1. Field of the Invention
This invention relates broadly to oilfield exploration. More particularly, this invention relates to apparatus and methods for expediting the downhole sampling of formation hydrocarbons.
2. State of the Art
One technique utilized in exploring a subsurface formation for oil is to obtain oil samples downhole. Various tools such as the MDT and the CHDT (trademarks of Schlumberger) tools are extremely useful in obtaining and analyzing such samples. Tools such as the MDT tool (see, e.g., U.S. Pat. No. 3,859,851 to Urbanosky, and U.S. Pat. No. 4,860,581 to Zimmerman et al., which are hereby incorporated by reference herein in their entireties) typically include a fluid entry port or tubular probe cooperatively arranged within one or more wall-engaging packers for isolating the port or probe from the borehole fluids, one or more sample chambers which are coupled to the fluid entry by a flow line having one or more control valves arranged therein, means for controlling a pressure drop between the formation pressure and sample chamber pressure, and sensors for obtaining information relating to the fluids. The sensors may include pressure transducers for monitoring the pressure of the fluid. In addition, optical sensors may be supplied by an OFA, CFA or LFA (all trademarks of Schlumberger) module (see, e.g., U.S. Pat. No. 4,994,671 to Safinya et al., U.S. Pat. No. 5,266,800 to Mullins, and U.S. Pat. No. 5,939,717 to Mullins which are hereby incorporated by reference herein in their entireties) in order to determine the make-up of the fluid being admitted into the tool, etc.
The CHDT tool is similar in various manners to the MDT tool, but is used when the borehole is cased with a casing. The CHDT tool includes a mechanism for perforating the casing such as a drilling mechanism (see, e.g., “Formation Testing and Sampling through Casing”, Oilfield Review, Spring 2002 which is hereby incorporated by reference herein in its entirety) and for plugging the casing after testing.
The MDT and CHDT tools in their normal applications are used to obtain formation oil samples with a low viscosity; typically up to 30 cp. In certain circumstances and with special adaptations, oils with a higher viscosity have been sampled. It is believed that the maximum viscosity that has been sampled using an MDT or CHDT tool is an oil having a viscosity of 3200 cp, but the sampling process often requires several adaptations and can take many hours.
It will be appreciated by those skilled in the art that exploitation of more viscous hydrocarbons is becoming increasingly important due to the depletion of conventional low viscosity hydrocarbon reserves. Sampling these oils for reservoir characterization is very challenging as oils with a higher viscosity have a low mobility and are hard to sample or cannot be sampled at all depending on the local circumstances. In fact, the low mobility of these oils often results in very long sampling times or makes it impossible to retrieve a sample. If sampling times are too long there is a chance that the tool can get stuck in the borehole.
While larger sampling ports on the sampling tool can improve the flow of oil into the sampling tool, the tool size and sealing concerns limit the maximum size of the sampling ports.